1. Field Of The Invention
The present invention relates to gas turbine engines, notably for aeronautical applications, comprising a low-pressure spool and a high-pressure spool. It is aimed at the layout of the bearings which, within the engine casing, support the LP and HP shafts and, more particularly, at the downstream bearings.
2. Description Of The Related Art
A twin-spool or two-body gas turbine engine comprises a first rotary assembly, known as the low-pressure (BP) spool, formed of a shaft connecting an LP compressor at the upstream end, upstream and downstream being defined with respect to the flow of air through the machine, and an LP turbine at the downstream end. Each of the compressor and turbine elements may be made up of one or more stages. The two LP elements are spaced axially apart and leave a space for a second rotary assembly known as the high-pressure (HP) spool, formed of an HP compressor located downstream of the LP compressor, and of an HP turbine located upstream of the LP turbine. The HP compressor and the HP turbine are mechanically connected to one another by a connecting member in the form of a drum. The combustion chamber of the engine, which is stationary with respect to the two spools, is annular and housed circumferentially around said drum. It receives compressed air from the compressors of the LP stages and of the HP stages in turn, and delivers high-energy combustion gases to the HP and LP turbine stages in turn. The engine may comprise a fan rotor at the front, driven by the shaft of the LP spool. Other layouts are known.
A known engine such as the CFM56 comprises structural casing elements notably supporting the rotary assemblies via bearings. On the upstream side, a casing element, known as the intermediate casing, comprises a hub supporting the LP shaft via an upstream LP bearing. On the downstream side, a casing element known as the exhaust casing also comprises a hub supporting the LP shaft via a downstream LP bearing. The HP spool is supported by the LP shaft on the downstream side using an inter-shaft bearing.
An engine mounted on an aircraft experiences transverse dynamic loadings when the aircraft undertakes direction-change maneuvers. With such a bearing layout and for engine embodiments notably in which the length is great with respect to the slenderness of the shaft of the low pressure LP spool, the present applicant has analyzed the behavior of the HP and LP rotors when the engine is subjected to such maneuvering loadings. The transverse movement of the rotors along the engine axis is a critical parameter inasmuch as this movement has a direct influence on how the clearances between the tips of the blades and the stator rings are taken up. These clearances have to be kept at low values if optimum performance is to be maintained.